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MiSawa/gist:4088861

Created November 16, 2012 16:46
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gistfile1.rb
p Hash.new{|hash, n|
hash[n] = n.zero? ? 1 : n < 0 ? 0 : (1..4).map{|i|hash[n-i]}.inject(&:+)
}[50]
#=> 100808458960497
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